Non-Thyroidal Illness Syndrome (NTIS)

(also called Euthyroid Sick Syndrome or Low T3 Syndrome)

Definition

Non-thyroidal illness syndrome (NTIS) is a state of altered thyroid hormone metabolism, regulation, transport, and action occurring during systemic illness, without intrinsic thyroid gland pathology.

✔ Thyroid gland structurally normal
✔ Abnormal TFTs reflect adaptive + maladaptive responses
✔ Common in ICU and critically ill patients

Terminology 

Term	Meaning
Euthyroid Sick Syndrome	Old umbrella term
Low T3 syndrome	Early/mild NTIS
Low T3–T4 syndrome	Severe/prolonged NTIS
Consumptive hypothyroidism	Rare extreme (↑ D3 in tumors, hemangiomas)

Typical Hormonal Pattern

Parameter	Change	Mechanism
Total T3	↓↓↓	↓ D1, ↑ D3
Free T3	↓↓↓	Reduced conversion + transport
Reverse T3 (rT3)	↑↑	Reduced clearance
Total T4	Normal → ↓	Severe illness
Free T4	Normal / ↓	Binding changes
TSH	Normal / ↓	Hypothalamic suppression

 Low T3 + high rT3 = hallmark biochemical signature

Pathophysiology 

 1️⃣ Deiodinase Dysfunction 

Enzyme	Change	Effect
Type 1 (D1)	↓	↓ T4 → T3
Type 2 (D2)	↓	↓ intracellular T3
Type 3 (D3)	↑	↑ T4 → rT3 (inactive)

 Central mechanism of NTIS

 2️⃣ Cytokine-Mediated Endocrine Suppression

Inflammatory cytokines:

• IL-1
• IL-6
• TNF-α
• IFN-γ

Effects:

• ↓ Hypothalamic TRH
• ↓ Pituitary TSH secretion & pulsatility
• ↓ Thyroid hormone receptor expression

 Explains normal or low TSH despite low T3/T4

 3️⃣ Altered Thyroid Hormone Binding

• ↓ Thyroxine Binding Globulin (TBG)
• ↓ Albumin
• ↑ Non-esterified fatty acids
• Drug displacement

Result:

• Total hormone levels misleading
• Free hormone assays may also be unreliable in ICU

 4️⃣ Impaired Cellular Uptake

• ↓ MCT8 & OATP1C1 transporters
• Reduced intracellular T3 despite normal serum levels

 Leads to tissue-level hypothyroidism

 5️⃣ Thyroid Hormone Resistance at Nuclear Level

• ↓ Thyroid hormone receptor (TRα, TRβ)
• Altered co-activator / co-repressor balance
• Reduced transcription of T3-dependent genes

 6️⃣ Hypothalamic–Pituitary–Adrenal (HPA) Axis Interaction

• ↑ Cortisol suppresses:
• TRH
• TSH
• Peripheral T4 → T3 conversion

 Explains overlap with critical illness–related corticosteroid insufficiency

Why Does NTIS Occur? (Adaptive vs Maladaptive)

 Early / Acute Phase (Adaptive)

• ↓ Basal metabolic rate
• ↓ Oxygen consumption
• ↓ Protein catabolism
• Energy conservation during stress

 Prolonged ICU / Chronic Critical Illness (Maladaptive)

• Muscle wasting
• Impaired wound healing
• Immunosuppression
• Myocardial dysfunction
• Poor neurological recovery

Clinical Settings Associated with NTIS

🔸 Acute Critical Illness

• Sepsis / septic shock
• ARDS
• Polytrauma
• Burns
• Acute MI
• Stroke / TBI
• Post-cardiac surgery
• Major abdominal surgery

🔸 Chronic Systemic Illness

• CKD (especially dialysis)
• Cirrhosis
• Advanced heart failure
• Malignancy
• Starvation / anorexia nervosa
• Prolonged ICU stay (>7–10 days)

Drug-Induced NTIS 

Drug	Mechanism
Dopamine	↓ TSH secretion
Glucocorticoids	↓ TRH, ↓ TSH, ↓ T4→T3
Amiodarone	↓ Deiodinase, iodine load
Propranolol	↓ Peripheral conversion
Iodinated contrast	↓ Deiodinase
Heparin	↑ Free T4 artefact

Severity-Based Biochemical Progression

Stage	TFT Pattern	Prognosis
Mild illness	↓ T3	Good
Moderate illness	↓ T3 + ↑ rT3	Variable
Severe illness	↓ T3 + ↓ T4	Poor
Prolonged ICU	↓ T3 + ↓ T4 + ↓ TSH	High mortality

 Low T4 is the strongest mortality predictor

NTIS vs True Hypothyroidism 

Feature	NTIS	Hypothyroidism
T3	↓	↓
T4	Normal / ↓	↓
TSH	Normal / ↓	↑
rT3	↑	↓
Goitre	#	Possible
Antibodies	#	Often +
Treatment	No	Yes

Recovery Phase – IMPORTANT PITFALL

• TSH may transiently rise (5–20 mIU/L)
• Mimics subclinical hypothyroidism
• Do NOT treat
• Recheck after 6–8 weeks

NTIS in ICU – Practical Interpretation

✔ Abnormal TFTs common (up to 70% ICU patients)
✔ TFTs do NOT guide therapy
✔ Measure TFTs only if thyroid disease suspected

Should NTIS Be Treated? (LATEST GUIDELINE CONSENSUS)

❌ Routine Thyroid Hormone Therapy – NOT Recommended

• No mortality benefit
• No ICU outcome improvement
• Risks:
• Arrhythmias
• Increased myocardial oxygen demand
• Catabolism

 Experimental settings only:

• Prolonged ICU (>2–3 weeks)
• Research protocols
• Selected pediatric congenital heart disease studies

📌 Standard of care = Treat underlying illness only

Prognostic Significance

• ↓ T3 correlates with:
• SOFA score
• APACHE II
• ICU mortality
• NTIS = marker of disease severity

When to Investigate for True Thyroid Disease in ICU

✔ Past thyroid disease
✔ Goitre / ophthalmopathy
✔ TSH >10 mIU/L
✔ TSH <0.01 mIU/L
✔ Persistent abnormal TFTs after recovery